There is an epidemic of obesity. We are all going prematurely to hell in a heavily reinforced handcart, the victims of our own self-indulgence and laziness. And a tsunami of diabetes, osteoarthritis, heart disease and cancers is about to hit us. Bring on the lettuce leaves and five-mile runs.

That's the orthodoxy, but a number of scholars are now accusing obesity researchers, the media and public health officials of exaggeration. This week sees the publication of yet another of these dissenting views. The Obesity Epidemic: science, morality and ideology (Routledge 2005) is the work of two Australian academics, Michael Gard and Jan Wright, experts in physical education and education respectively. I can hear the sniffs of disapproval already.

They are outside the medical community, but that doesn't mean they have nothing of value to say. Scientists talk about fat as if it had a clear cause. Too much energy in and not enough out. The reality, these authors claim, is that there are many aspects of both the causes and the consequences of being overweight or obese which are not explained by this simplistic model.

The authors do not dispute that the proportion of those who are obese in the population has roughly doubled in the US and many parts of Europe since 1980. Nor do they dispute the increase in cases of Type 2 diabetes. In the US, 55% of adults with diabetes are obese, compared with 31% in the general population, which surely links obesity and diabetes.

It is said that 365,000 Americans die prematurely each year because of obesity. This figure became written in stone almost as soon as it appeared in 1999. But it was derived from 10-year- old data, taken in part from self- reported weights and heights. In April, research published in the Journal of the American Medical Association by Katherine Flegal, a scientist at the Centres for Disease Control, which used newer data, showed that it is far from certain that there is measurable excess mortality among obese or moderately overweight Americans.

Many will say that this is because of better drug treatments. But 30-50% of prescription drugs are not taken as directed, and many lower income Americans don't have health insurance and can't afford drugs, so this doesn't ring true. If it was, one would think the pharmaceutical industry would have been shouting it from the rooftops, yet Flegal's findings did not make the front pages like the 1999 study. But given the $46bn weight loss industry dependent on scaring stouter citizens, that is hardly surprising.

You could regard all this in the same vein as global warming dissent - to be trashed either because you think it wrong, or think there is truth to the argument but it shouldn't be made because it encourages complacency.

But Gard and Wright have some points on the ideology surrounding obesity and also the way body mass index (weight divided by the square of height) is used as a measure. In the past, only those whose BMI was in excess of 30 were deemed overweight or obese. That value has now been changed to 25 and above, instantly exploding the number of obese people, labelling them as "abnormal" and in need of medical treatment.

This arbitrary definition of fatties vs thinnies has meant constant exhortation to get to your "ideal weight", within the magic 20-24 BMI band. Since there are naturally a range of weights within a population, many are trying to achieve the impossible. When they fail, they are told they are "weak-willed".

Obesity is one of those areas where science meets culture full on. Science has always been used by some as a church to which they retreat for factual underpinning of their moral beliefs. Fat people are bad, fat people are lazy, fat people are symptomatic of our moral decline.

To be overweight is said to be the product of a breakdown in family values and of slothful kids. Low-income groups are more likely to be obese, and so it is assumed that children living in poverty must watch more TV than rich children. But telling parents to restrict TV viewing is easier than dealing with health inequalities caused by poverty, a more likely cause of obesity than daytime TV, no matter how damaging that might seem.

We are right to be concerned about the rising levels of Type 2 diabetes but should we be worrying about the health futures of people with a BMI of 26 or 28 who are taking regular exercise? Probably not, but we will continue to tell these people that they are porky slobs who ought to try harder. In doing so, it is likely that we are condemning them to the sort of yo-yo dieting that results in weight gain, not weight loss.